Urea-formaldehyde and phenol-formaldehyde condensation resins are the major types of thermosetting resins currently used as adhesives for binding together solid lignocellulosic materials, including wood, to form useful products. The resins generally are provided as aqueous solutions containing high solid levels or as powders which may be dissolved in water or applied directly to the lignocellulosic particles prior to hot pressing. Even though most of the formaldehyde reacts with the urea or the phenol and forms part of the polymeric adhesive resin, some free formaldehyde remains and volatilizes from the final product into the immediate environment. Liberation of formaldehyde is also caused by hydrolysis of the resin. Formaldehyde has been implicated as a carcinogen and its presence in the environment poses a potential public health problem. Therefore, it is desirable to have suitable adhesives that do not contain formaldehyde as an ingredient.
The basic raw materials used in condensation resins are derived from petroleum and natural gas. Since the prices of these non-renewable resource materials have increased, attempts have been made to replace them with less expensive carbohydrates. For example, degradation products of sugars and starches have been employed as ingredients in binders for solid lignocellulosic materials to produce water proof plywood and particle boards. The Stofko, U.S. Pat. Nos. 4,107,379; 4,183,997 and 4,357,194 describe the use of such carbohydrates.
The Stofko U.S. Pat. No. 4,107,379 describes the application of a mixture of a sugar solution and an inorganic acid onto the surfaces of the lignocellulosic material to be bonded, followed by heating and pressing until the carbohydrates are transformed into furan-type compounds which act as adhesives.
The Stofko U.S. Pat. No. 4,183,997, which is an extension of the above patent, teaches the further addition of an alkaline agent to prevent an excessive decline in the pH of the resulting adhesive which upon heating can result in hydrolytic degration of wood. The patent states that the sugars may be added directly to the surfaces of the lignocellulosic materials as powders or first mixed with a liquid carrier along with a catalyst to accelerate the transformation of the sugars into water insoluble products and applied as a liquid. When the sugars are added as powders, the process requires melting the mixture and spreading it while it is still hot. This can result not only in the evolution of toxic HCl gas, but also clumping problems unless the lignocellulosic materials being bonded are kept at relatively high temperatures during the application of the hot melt. When a liquid is used as a carrier (water or ethanol) the solution containing the mixture of sugar(s) and catalyst is either applied directly to the materials to be bonded or it is preheated at a temperature from 100.degree. C. to 130.degree. C. for a period from 30 to 120 minutes prior to application.
The Stofko U.S. Pat. No. 4,357,194 describes adhesive compositions which contain, in addition to starches, phenolic containing materials, such as lignin, which are activated by contact with live steam.
The Gibbons et al. U.S. Pat. Nos. 4,085,075 and 4,085,076 describe methods for the manufacture of carbohydrate based thermoset resins. The former patent describes the addition of a carbohydrate and an aminoplast to a phenolformaldehyde novolak resin for making solid molding compositions. The latter patent describes the reaction of an aldose saccharide (preferably a hexose) with a phenolic compound and urea to form a liquid fusible resin. The liquid resin is then mixed with formaldehyde and the pH raised to 8.0 or more prior to the second stage reaction to form a resol resin. Both of the above patents require the presence of formaldehyde in the resin compositions to bind lignocellulosic particles. The formaldehyde is replaced in part by carbohydrate(s), but some formaldehyde is still employed.